Color television systems use a color subcarrier in the megahertz range which is modulated by a signal representing the color hue. The use of this alternating current subcarrier causes a pattern of bright spots to appear in the video picture, each bright spot corresponding to a peak of the color subcarrier wave. In normal commercial television, this pattern is filtered out, but in the process the sharpness and resolution of the picture are somewhat degraded.
Video images in such specialized applications as video-to-film conversions and large-screen video displays require a resolution which does not normally permit the use of such pattern filters, or at least restricts the breadth and depth of the filter. At the same time, these applications require a very high color fidelity.
Noise reduction and luminance-chroma separation techniques such as those described in U.S. Pat. No. 4,107,736 issued Aug. 15, 1978 and entitled "Noise Reduction System For Video Signals" have been developed to eliminate, among other things, the objectionable pattern without the use of a filter. Noise reduction systems of this type rely on the fact that in the NTSC system of television, the color subcarrier in adjacent frames of the television signal is 180.degree. out of phase. This allows the bright spots caused by the carrier to be cancelled out by an additive process. Inherently, this type of noise reduction system cannot cancel out patterns appearing in the moving portions of the picture. It is therefore desirable to make the pattern as unobtrusive as possible.
The human eye has a psychological tendency to concentrate on vertical and horizontal patterns much more strongly than on patterns lying at an angle. It is therefore desirable to make the pattern as fine as possible, and to make it move at a 45.degree. angle for minimum visual distraction.
The other problem in high-quality specialized television applications as mentioned herein is color fidelity. In the NTSC system, the color hue is quite sensitive to phase distortions. Most transmission media, unfortunately, tend to have somewhat nonlinear phase shift characteristics. Therefore, good hue correction is quite difficult in the NTSC system.
The PAL system achieves much better hue fidelity by reversing one of the color axes at a line rate. This causes phase distortions in adjacent lines to be equal but in opposite directions, so that the integration provided by the human eye produces a visual impression of the correct hue. Unfortunately, this color axis reversal makes it necessary to choose a color subcarrier frequency such that adjacent frames cannot have a 180.degree. phase difference between their color subcarriers. Consequently, equipment using the hue-correction abilities of the PAL system cannot be used with noise reduction equipment of the type described.